* blockstore: atomize slot clearing, relax parent slot meta check
clear_unconfirmed_slot can leave blockstore in an irrecoverable state
if it panics in the middle. write batch this function, so that any
errors can be recovered after restart.
additionally relax the constraint that the parent slot meta must exist,
as it could have been cleaned up if outdated.
* pr feedback: use PurgeType, don't pass slot_meta
* pr feedback: add unit test
* pr feedback: refactor into separate function
* pr feedback: add special columns to helper, err msg, comments
* pr feedback: reword comments and write batch error message
* pr feedback: bubble write_batch error to caller
* pr feedback: reword comments
Co-authored-by: steviez <stevecz@umich.edu>
---------
Co-authored-by: steviez <stevecz@umich.edu>
cc4072bce8